Millimeter polarimetry of star-forming regions: magnetic field structure around low-mass stars

We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure.     

Millimeter continuum measurements of circumstellar dust around very young low-mass stars

We investigate the question of disk formation during the protostar phase. We build on the results of Keene and Masson (1990) whose analysis of L1551 showed the millimeter continuum emission comes from both an unresolved circumstellar component, i.e., a disk and an extended cloud core. We model the dust continuum emission from the cloud core and show how it is important at 1.3 mm but negligible at 2.7 mm. Combining new 2.7 mm Owens Valley Interferometer data of IRAS-Dense cores with data from the literature we conclude that massive disks are also seen toward a number of other sources. However, 1.3 mm data from the IRAM 30 m telescope for a larger sample shows that massive disks are relatively rare, occurring around perhaps 5% of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. At 1.3 mm the median flux of IRAS-Dense cores is nearly the same as T Tauri stars in the sample of Beckwithet al. (1990). We conclude that the typical disk mass during the embedded phase is nearly the same or less than the typical disk mass during the T Tauri phase.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Statistical results from a jcmt survey of MM and sub-MM emission in herbig ae/be stars

We have detected 1.1 mm continuum emission from 24 of 53 Herbig Ae/Be stars surveyed with the JCMT. Survival analysis shows that 1.1 mm luminosity is correlated with bolometric luminosity and with IRAS 25µm luminosity. For those stars that were also detected at 0.45 or 0.8 mm we find a typical flux dependence of the form S _#x03BD; ³, which is steeper than that of most classical T Tauri stars.     

X-ray Activity and Accretion in Young Stellar Objects

I discuss recent observational results on the X-ray properties of young stellar objects, based mostly on Chandra and XMM-Newton observations. The sensitive X-ray data on large, well characterized samples of T Tauri stars (and a number of protostars) allow to study in detail the dependence of magnetic activity on the bulk properties of the young objects and to draw important clues towards the origin of the X-ray emission. The absence of a relation between X-ray activity and rotation for T Tauri stars clearly suggests that their magnetic activity cannot be simply explained by the action of a scaled-up solar-like dynamo. I discuss alternative models for the generation of magnetic fields and also consider the long standing question whether the X-ray properties of the T Tauri stars are related to the presence/absence of circumstellar disks or active accretion.     

原行星盘的研究进展

原行星盘是环绕在年轻星天体(如T Tauri型星,HAe/Be星)周围的气体尘埃盘,是具有初始角动量的分子云核在塌缩形成恒星过程中的自然结果,是行星系统的起源地。原行星盘研究不仅是恒星形成理论的重要组成部分,而且是行星形成理论的基础。首先介绍了盘的形成与演化规律;然后介绍了年轻星天体的能谱分布,盘的模型和参数(质量吸积率、质量、尺度、温度、寿命);随后讨论了尘埃颗粒在盘中生长的观测证据以及行星在盘中形成的大致过程;最后对原行星盘研究的现状和未来做了总结与展望。     

New Hα objects in dark nebulae. I.

Results of a search for new H objects in the Cepheus region are presented. The observations on which the search was based were made in 1979 and 1985 at the 40 Schmidt telescope of the Byurakan Astrophysical Observatory with a 4° objective prism. Of the 80 emission stars detected, 68 are new. Most of them are fainter than the sun in absolute magnitude. A large fraction of the emission stars discovered may be flare stars as well as T Tau and Herbig Ae/Be stars.Translated fromAstrofizika, Vol. 39, No. 1, pp. 57–65, January–March, 1996.     

Radial migration of planetesimals

Planetesimals orbiting a protostar in a circumstellar disk are affected by gravitational interaction among themselves and by gas drag force due to disk gas. Within the Kyoto model of planetesimal accretion, the migration rate is interpreted as the inverse of the planetary formation time scale. Here, we study time scales of gravitational interaction and gas drag force and their influence on planetesimal migration in detail. Evaluating observations of 86 T Tauri stars (Beckwithet al., 1990), we find the mean radial temperature profile of circumstellar disks. The disk mass is taken to be 0.01M in accordance with minimum mass models and observed T Tauri disks. The time scale of gravitational interaction between planetesimals is studied analogously to Chandrasekhar's stellar dynamics. Hence, Chandrasekhar's coefficient , defined as the fraction between the mean separation of planetesimals and the impact parameter, plays an important role in determining the migration rate. We find ln to lie between 5 and 10 within the protosolar disk. Our result is that, at the stage of disk evolution considered here, gas drag force affects the radial migration of planetesimals by a few orders of magnitude more than gravitational interaction.Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

10-µm images and spectra of T Tauri stars

T Tauri stars are young stars usually surrounded by dusty disks similar to the one from which we believe our own Solar System formed. Most T Tauri stars exhibit a broad emission or absorption band between 7.5 and 13.5µm which is attributed to silicate grains in the circumstellar environment. We imaged three spatially resolved T Tauri binaries through a set of broadband filters which include the spectral region occupied by the silicate band. Two of these objects (T Tauri and Haro 6–10) are infrared companion systems in which one component is optically much fainter but contributes strongly in the infrared. Both infrared companions exhibit a deep silicate absorption which is not present in their primaries, indicating that they suffer very strong local extinction which may be due to an edge-on circumstellar disk or to a dense shell. We also took low resolution spectra of the silicate feature of two unresolved T Tauris to look for narrow features in the silicate band which would indicate the presence of specific minerals such as olivine. We observed GK Tau, for which Cohen and Witteborn (1985) reported a narrow emission feature at 9.7µm, but do not find evidence for this feature, and conclude that it is either time-dependent or an artifact of absorption by telluric ozone.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

A near infrared speckle imaging study of T Tauri Stars

The results of a speckle imaging survey of T Tauri stars suggest that most, if not all, young low mass stars have companions. Repeated observations of these young binary stars have revealed orbital motion in the closest pairs (0.3), proving that these systems are indeed gravitationally bound and providing the basis for mass estimates in the upcoming years. These mass estimates are necessary to distinguish between the various binary star formation mechanisms that have been proposed to date.     

Location of type I radio continuum and bursts on Yohkoh soft X-ray maps

A solar type I noise storm was observed on 30 July, 1992 with the radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA) and the soft X-ray (SXR) telescope on board theYohkoh satellite. The spectrogram was used to identify the type I noise storm. In the VLA images at 333 MHz a fully left circular polarized (100% LCP) continuum source and several highly polarized (70% to 100% LCP) burst sources have been located. The continuum and the bursts are spatially separated by about 100 and apparently lie on different loops as outlined by the SXR. Continuum and bursts are separated in the perpendicular direction to the magnetic field configuration. Between the periods of strong burst activities, burst-like emissions are also superimposed on the continuum source. There is no obvious correlation between the flux density of the continuum and the bursts. The burst sources have no systematic motion, whereas the the continuum source shows a small drift of 0.2 min^–1 along the X-ray loop in the long-time evolution. The VLA maps at higher frequency (1446 MHz) show no source corresponding to the type I event. The soft X-ray emission measure and temperature were calculated. The type I continuum source is located (in projection) in a region with enhanced SXR emission, a loop having a mean density of n _e = (1.5 ± 0.4) × 10⁹ cm^–3 and a temperature ofT = (2.1 ± 0.1) × 10⁶ K. The centroid positions of the left and right circularly polarized components of the burst sources are separated by 15–50 and seem to be on different loops. These observations contradict the predictions of existing type I theories.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Astrometric investigation with the Tautenburg Schmidt telescope

The observations and the plate reduction technique for the determination of positions and absolute proper motions which is used in Potsdam are described. Recent results have shown that an accuracy of about 0 _. 1 for positions and 0 _. ⁷ cent _. ^–1 for proper motions can be achieved both for bright (8^m–12^m) and faint (16^m–18^m) stars. Three astrometric programmes using the Tautenburg plates are presented.     

T Tauri stars: Physical parameters and evolutionary status

Long-term homogeneous photometry for 35 classical T Tauri stars (CTTS) in the Taurus–Auriga star-forming region has been analyzed. Reliable effective temperatures, interstellar extinctions, luminosities, radii, masses, and ages have been determined for the CTTS. The physical parameters and evolutionary status of 35 CTTS from this work and 34 weak-line T Tauri stars (WTTS) from previous studies have been compared. The luminosities, radii, and rotation periods of low-mass (0.3–1.1 M _⊙) CTTS are shown to be, on average, greater than those of low-mass WTTS, in good agreement with the evolutionary status of these two subgroups. The mean age of the younger subgroup of WTTS from our sample (2.3 Myr) essentially coincides with the mean duration of the protoplanetary disk accretion phase (2.3 Myr) for a representative sample of low-mass stars in seven young stellar clusters. The accretion disk dissipation time scale for the younger subgroup of CTTS (<4 Myr) in the Taurus–Auriga star-forming region is shown to be no greater than 0.4 Myr, in good agreement with the short protoplanetary disk dissipation time scale that is predicted by present-day protoplanetary disk evolution models.     

On mechanisms separating stars into normal and chemically peculiar

The paper argues in favor of the assumption that magnetic and non-magnetic protostars, from which CP stars were formed, are the objects that had rotation velocities of the parent cloud V smaller than a critical value V _c . At V greater than the critical value, differential rotation emerges in the collapsing protostellar cloud, which twists magnetic lines of force into an’ invisible’ toroidal shape and disturbs the stability of the atmosphere. In magnetic protostars, the loss of angular momentum is due to magnetic braking, while in metallic protostars, the loss of rotation momentum occurs due to tidal interactions with a close component. HgMn stars are most likely not affected by some braking mechanism, but originated from the slowest protostellar rotators. The boundary of V _c where the differential rotation occurs is not sharp. The slower the protostar rotates, the greater the probability of suppressing the differential rotation and the more likely the possibility of CP star birth.     

Mass-luminosity distribution and physical conditions of ionized gas in NGC 4575

Photographic photometry and spectroscopic observations of NGC 4575 suggest it to be a galaxy of reduced dimensionsD×d=14.4×13.5 kpc and of high luminosityM=–20.7. The rotation curve was also determined. Assuming a model of three homogeneous similar spheroids, we derived the density and mass distribution, and their total mass was found to be _T =2.33 × 10¹⁰.The mass luminosity ratio does not vary withinr=24, indicating that the stellar composition is similar within it, and the ratio _T / _T 1 suggests that this object contains a high proportion of young stars. From the emission lines it is found that the electronic density Ne100 cm^–3 is relatively low in the HII regions. The abundance ratiosN(N)/N(S) andN(N)/N(H) for the nucleus and two emission regions were also derived.     

Gravitational collapse of cosmic gas clouds and formation of star clusters

In this paper the gravitational collapse of cosmic gas clouds and formation of star clusters has been considered. Hoyle's view of successive fragmentation has been taken as the basic mechanim in the present work. The initial masses of protostars have been estimated as the function of their distances from the centre of the cluster and the intensity of the magnetic field of the medium. It has been shown that the fragmentation process is greatly inhibited by the presence of a strong magnetic field. A model has been constructed showing how a protostar grows in mass by accretion from the surrounding medium, on the basis of the assumption that as the star moves at random in the cluster it picks up a fraction of the material through which it passes. It has been estimated that a protostar of initial mass of about 0.1M grows to one of 1.0M in a time period which ranges from a few multiples of 10⁵ to a few multiples of 10⁷ yr, depending on the parameters involved in the accretion process. The number of stars per unit mass range has also been estimated; it is found to be proportional tom ^–3.3,m being the mass of the star.     

A new photometric study of T Tauri stars in the infrared

We have compiled infrared photometric data from the literature of practically all T Tauri stars found up to date including 444 classical T Tauri stars (CTTSs), 1698 weak-line T Tauri stars (WTTSs) and 1258 not classified T Tauri stars (3400 in total) in addition to 196 post-T Tauri stars (PTTSs). From this data bank we extract the infrared characteristics of the different groups and discuss different origins of the infrared radiation. The observational data are taken from the AKARI, IRAS, WISE and 2MASS missions. We show that in the wavelength range 1–140 μm, all T Tauri stars have infrared excesses. CTTSs have more infrared excess than WTTSs, while PTTSs have little or no infrared excess. We found that in the 1–3 μm wavelength range the infrared emission of T Tauri stars is mainly due to thermal radiation from the photosphere and hot dust grains from circumstellar envelopes. In the 3–140 μm wavelength range the infrared emission of T Tauri stars is mainly due to radiation from dusty/gaseous disks surrounding the stars. In addition, we also make a comparison between T Tauri stars and Herbig AeBe stars (HAeBe). There are some differences between these two kinds of objects in that for HAeBe stars the infrared radiation as a rule originates in dusty/gaseous disks in the 1–3 μm wavelength range, while in the range 3–12 μm it is possibly due to PAH emission for about half of HAeBe stars. In other wavelength ranges both kinds of stars have similar infrared characteristics indicating emission from dusty/gaseous disks.     

Radial pulsation of less-massive yellow supergiants

The conclusions of the present paper broadly are: (a) The galactic concentration of doubles by comparing the distributions in galactic latitudes 0°<20° and >40° is nearly twice as large as the galactic concentration of stars in general. (b) The astrographic catalogues are not complete in the fainter magnitudes. (c) The large value of the ratioT:O _k (observed to optical number of pairs) from Kreiken's formula shows that almost all stars in the group 0<d5 and quite a few in the other two groups, viz., 5<d10 and 10<d15 might be shown true binaries. Consequently, Aitken's working definition of a true binary should be extended if it were to include all true binaries. (d) The doubles are probably stars of Population I. (e) The logarithm to the base 10 of the cumulative counts can be represented by an empirical relationA+B(m–1.5)+C(m–1.5)².Communication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wilhelm Bessel (1784–1846)     

Characteristics of Mauna Kea relating to the Japan National Large Telescope

The characteristics of the Mauna Kea site are reviewed. An extensive site survey by NOAO during 1984–1985 showed that the average seeing was in the 0.4–0.6 range and thatr ₀ was approximately 30 cm. An image quality study at the UH 88-inch telescope in 1987 showed the free atmosphere seeing was 0.5 and the boundary layer contributed less than 0.25.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

A morphological study of the solar granulation

Some properties of the photospheric granulation near the centre of the quiet sun have been investigated on the basis of two high-definition Stratoscope photographs. We obtain (1) a number density of 54 granules per 10 × 10, (2) a total number of granules on the whole surface of 6.3 × 10⁶, (3) a mean cell diameter of 1.5 or 1100 km and a mean granule diameter of 1.2 or 850 km, the difference of 250 km being ascribed to the mean width of the dark intergranular lanes, (4) frequency distributions of cell diameters and granule diameters (Figures 1 and 2), and (5) an isophotal map in relative intensity (Figure 3). In general, larger granules are brighter than smaller granules (Figure 4). Cross-section profiles are shown for some granules and intergranular lanes (Figures 5 and 6). These quantities have not yet been corrected for the finite resolution of the telescope.     

Potential protostars in cloud cores

We present H₂CO observations of young protostar candidates in the Serpens Cloud Core. We find evidence for dense molecular gas in the cores of these objects that is warmer than the surrounding dust. The strong emission and gas properties support the premise that many of these sources may be very young protostars.